Tunable circuits such as a voltage-controlled oscillator (VCO) in a phase-locked loop (PLL) often obtain their tuning using the variable capacitance from a varactor. The variation of a tuning voltage for the varactor varies its capacitance. For example, the feedback within a PLL adjusts the tuning voltage for the varactor (or varactors) so that the desired output frequency is produced by the VCO. Ideally, a fixed tuning voltage produces a fixed capacitance for the varactor to operate the varactor in a high gain region of its tuning curve. But even with a fixed tuning voltage, the capacitance for a varactor will increase with temperature so as to have a proportional-to-absolute-temperature (PTAT) profile. This increase in capacitance forces the VCO to have an output frequency that is lower than what was desired. An example error for the VCO frequency would be −700 KHz per degree Celsius. The temperature-induced increase in varactor capacitance can thus push the VCO frequency off from the desired output frequency by 100 MHz or more depending upon the temperature increase or decrease.
Given the varactor's temperature sensitivity, is thus conventional to compensate the tuning voltage for a varactor to reduce its temperature sensitivity. But prior art temperature compensation schemes have proven to be problematic. For example, it is conventional for a bias circuit to use the threshold voltage of a bipolar junction transistor (BJT) to provide the tuning voltage for a varactor. The threshold voltage has a complementary-to-absolute-temperature (CTAT) profile so the tuning voltage is reduced as the temperature increases to offset the varactor's PTAT capacitance behavior. But the threshold voltage for a BJT is process independent. As the power supply voltage in modern designs has dropped (e.g., to 675 mV), it becomes difficult to design a BJT-based bias circuit for a varactor since the threshold voltage of a BJT is typically around 700 mV. To lower the BJT threshold voltage for such reduced power supply voltage bias circuits requires the BJT size to be increased dramatically, which lowers density for the bias circuit. In addition, the sensitivity to ground bounce is also more pronounced as the power supply voltage is reduced.
To increase density at lower supply voltages, a current-source-based bias circuit has been developed in which the tuning voltage for a negative terminal of the varactor is developed by a difference current equaling the difference between a current from a PTAT current source and a temperature-constant current from a bandgap current source. As the temperature increases, the tuning voltage reduces to compensate the temperature drift that would otherwise occur for the varactor. But the matching of the current sources is difficult and adds to design complexity and cost.
Accordingly, there is a need in the art for improved varactor temperature compensation, particularly at low power supply voltages.